As general pressure detection devices of this kind, there have been conventionally proposed devices each of which is constructed of a sensing part that outputs an electric signal responsive to an applied pressure, a pressure receiving diaphragm that receives the pressure, and a pressure transmission part that is interposed between the sensing part and the pressure receiving diaphragm and transmits the pressure received by the pressure receiving diaphragm to the sensing part (for example, JP-A-5-34231).
In the pressure detection device of this kind, three parts of the sensing part, the pressure receiving diaphragm, and the pressure transmission part are put into contact with each other in a state where the pressure receiving diaphragm and the sensing part sandwich the pressure transmission part to previously apply load, that is, to apply a pre-load to the pressure transmission part to thereby construct a pressure transmission mechanism.
In this pressure detection device, the pressure received by the pressure receiving diaphragm is transmitted to the sensing part via the pressure transmission part. Then, the sensing part outputs an electric signal responsive to the applied and transmitted pressure. Accordingly, this pressure detection device can detect pressure.
The inventor of the present application has made and studied a prototype of the pressure detection device of this kind and has found the following problems arising from the device.
The pressure detection device of this kind, as shown in FIG. 12, is used as a combustion pressure sensor that is provided in an engine block 200 as an attachment portion of a vehicle and detects pressure in a combustion chamber 202 (cylinder pressure).
A housing 210 of a pressure detection device 210 is provided with a detection element 230 as a sensing part that outputs an electric signal responsive to an applied pressure.
This detection element 230 is constructed of a semiconductor chip, for example, and has a strain gauge function such that the detection element 230 itself is strained to output a signal responsive to a detection pressure on the basis of its strain.
In the example shown in FIG. 12, this detection element 230 is supported by a hollow cylindrical metal stem 220, whose one end is an opening portion 221 and whose other end is closed by a thin diaphragm 222, and is fixed by fused glass or the like to the outer surface of the diaphragm 222 of the metal stem 220 as this supporting part.
The metal stem 220 is inserted into the housing 210 such that the diaphragm 222 is faced toward the inside of the housing 210, and the metal stem 220 and the housing 210 are joined and fixed to each other by welding, bonding, or the like.
As shown in FIG. 12, one end portion of a metal case 216 as a cylindrical part formed of metal in a cylindrical shape is joined and fixed to the opening portion 221 of the metal stem 220 at the tip of the housing 210. In other words, the detection element 230 as a sensing part is coupled to the one end portion of the metal case 216 via the metal stem 220.
A pressure receiving diaphragm 215 that receives pressure and is formed of metal in the shape of a circular plate is provided on the other end portion of the metal case 216. Here, the metal case 216 is joined and fixed to the pressure receiving diaphragm 215 by welding such as laser welding.
Accordingly, the pressure receiving diaphragm 215 and the metal stem 220 are integrated into one unit via the metal case 216. Then, this pressure receiving diaphragm 215 faces the above-described combustion chamber 202 and receives combustion pressure (cylinder pressure), thereby being strained.
As shown by arrows in FIG. 12, the one end portion of the pressure transmission part 217 is put into contact with the detection element 230 via the diaphragm 222 of the metal stem 220 in such a manner that a pre-load is applied to the detection element 230, and the other end portion of the pressure transmission part 217 is put into contact with the pressure receiving diaphragm 215 in such a manner that a pre-load is applied to the pressure receiving diaphragm 215.
In the pressure transmission mechanism constructed of the pressure receiving diaphragm 215, the pressure transmission part 217, and the detection element 230, the pressure in the combustion chamber 202, that is, the subject pressure is applied to the diaphragm 222 of the metal stem 220 and the detection element 230 from the pressure receiving diaphragm 215 via the pressure transmission part 17.
To prevent a pressure leakage from the combustion chamber 202, a tapered surface 223 is formed on the outer peripheral surface of the metal stem 220 provided with the detection element 230 in the pressure detection device, and this tapered surface 223 is pressed onto and put into contact with a tapered surface formed on the threaded hole portion 201 side to thereby seal the engine block 200.
In the pressure detection device shown in FIG. 12, the pressure diaphragm 215 has a spring function. By spring characteristic exerted by the pressure receiving diaphragm 215, the pre-load between the pressure receiving diaphragm 215, the pressure transmission part 217, and the detection element 230, that is, the pre-load between the parts constructing the pressure transmission mechanism is held, and fluctuations in the pre-load caused by the difference in thermal expansion between these parts are absorbed.
However, when the pressure receiving diaphragm 215 has the function of receiving pressure, the function of holding a pre-load, the function of absorbing fluctuations in the pre-load caused by the difference in thermal expansion, and the function of transmitting the pressure, and further is exposed directly to a measurement environment, there are presented the following problems.
First, in the case where the pressure detection device is used for a combustion pressure sensor or the like, when flame develops in the combustion chamber 202 at the time of combustion, the pressure receiving diaphragm 215 exposed to the measurement environment is directly irradiated with the flame.
Therefore, the pressure receiving diaphragm 215 is heated and hence softened by the flame when it is irradiated with the flame. In this case, linearity in the spring characteristic of the pressure receiving diaphragm 215 is deteriorated thereby losing the spring characteristic.
In the pressure receiving diaphragm 215, a pre-load is applied to the center portion of the pressure receiving diaphragm 215 by the pressure transmission part 217. Accordingly, when an excessive load is applied to the pressure receiving diaphragm 215, it causes damage to linearity in the spring characteristic of the pressure receiving diaphragm 215.
Moreover, there is a possibility that a temperature change caused by thermal shock or the like will cause damage to linearity in the spring characteristic of the pressure receiving diaphragm 215 at the time of low temperatures or high temperatures.
The pressure receiving diaphragm 215 is exposed to the combustion chamber 202 that is the measurement environment and hence deposits such as soot produced in the combustion chamber 202 adhere to the surface of the pressure receiving diaphragm 215.
Then, the adhesion of the deposits to the pressure receiving diaphragm 215 varies the spring characteristic of the pressure receiving diaphragm 215 from the initial state or causes damage to the spring characteristic.
Various problems such as a problem that linearity in the spring characteristics of the pressure receiving diaphragm 215 is damaged are raised by heating, the adhesion of the deposits, and the excessive application of load. In short, the spring characteristic acting on the pressure transmission mechanism constructed of the pressure receiving diaphragm 215, the pressure transmission part 217, and the sensing part 30 fluctuates and has a bad effect on the sensor characteristics.
When the pressure detection device is mounted on the engine block 200, the tapered surface 223 as a sealing surface formed on the metal stem 220 as a supporting part is pressed onto the inner surface of the threaded hole portion 201 of the engine block 200 to seal the engine block 200.
In this case, there is a possibility that the metal stem 220 as a part having the sealing surface 223 is pressed onto the threaded hole portion 201 and hence may be deformed.
When the metal stem 220 is deformed, its deformation is transmitted as mechanical noises to the detection element 230 mounted directly on the metal stem 220, thereby deteriorating the sensing characteristics of the detection element 230.